110 THE MICnOSCOPE AND HOW TO USE IT. 



it passes through the capillaries. Again ; the gelatine for the 

 arteries may be coloured with coarser pigments, such as Prussian 

 blue or ultramarine. The latter furnishes a beautiful blue. A 

 plaster of Paris mass injected after a gelatine mass will drive it on 

 until the plaster reaches the smallest vessels, thus producing a 

 double injection. The usual method of double injections is to 

 first inject a gelatine mass of one colour into the artery until 

 increasing pressure gives notice that a mass is entering the capil- 

 laries, and immediately after inject a different coloured mass into 

 the vein. A better way is to fill both vessels at the same time and 

 under exactly the same pressure. The pressure is kept low at the 

 beginning, so that all the arteries and veins shall be thoroughly 

 filled before either mass begins to enter the capillaries. As the 

 pressure is increased, the different masses meet each other in the 

 capillaries, and, if the pressure on each is equal, the vessels may 

 be filled as full as compatible with safety, without danger of either 

 colour being driven from one set of vessels into the other. This 

 will be better understood by referring to the drawing (PI. XIV., Fig. 

 3). The desired pressure is secured by letting a stream of water 

 from a hydrant or elevated vessel flow into a tight vessel. As the 

 water flows in, the air is forced out through the rubber tube. A, 

 into the wide-mouthed bottle, F ; through the close-fitting cork are 

 two other glass tubes. Those extend below just through the cork, 

 and above connect respectively with the rubber-tubes, C or D. 

 Into the side of F, near the bottom, is fitted another tube, E, 

 reaching to a height of ten inches or more, open above, and gra- 

 duated into inches. If preferred, this tube may also pass through 

 the cork and extend down well into the mercury, with which F is 

 partly filled. B contains a blue injection mass for filling veins, 

 and R, a similar bottle, a red mass for arteries. The interiors of 

 these bottles are connected with bottle F by tubes D and C. 

 Each of the bottles B and R has a tube, which, starting from near 

 the bottom, passes through the cork, and is, a little above this, 

 bent at right angles. Connected with these are rubber tubes, H 

 and I. When the water is allowed to flow into the reservoir, the 

 air is forced out through A into F, thence along tubes D and C 

 into B and R. As soon as the pressure in these bottles is suffi- 

 ciently great, the liquid which they contain will be driven out 



